Reply to ASK-AN-EARTH-SCIENTIST

Subject: Earthquake potential for Vancouver Island

I have a question regarding the big earthquake that is supposed to
occur in southern British Columbia, Canada, and the Washington/Oregon
areas of the USA. I live in Qualicum Beach on Vancouver Island, BC,
Canada and have heard rumors that our island will either "break off and
sink??", or "split in two at the Port Alberni Inlet". Could you
attempt to clear this up for me?

"Supposed to occur" is putting things a bit strongly. The people at the
Pacific Geoscience Centre near Sidney have ascertained that the outer
section of the Cascadia margin, all the way from Vancouver Island
almost to Cape Mendocino, has no earthquakes at all an therefore
appears to be locked. If so, strain is building up there and that is
the section that will rupture to give the next big Cascadia earthquake.
But nobody can yet give much of an idea when the next big event will
occur. We know the last one was in January 1700 and that they occur
roughly every 500 years, but the time between major earthquakes could
be as little as 200 years or as much as a thousand. On the other hand,
when an earthquake comes it's likely to be very large: the 1700 event
was over magnitude 9, about as large as the Alaska earthquake of 1964.
That means severe shaking lasting for several minutes, a threat that is
not to be taken lightly.
Will part of Vancouver Island "break off and sink," or "split in two at
Alberni Inlet?" No. In a big earthquake the seafloor offshore from
Vancouver Island will be uplifted and the area along the coast will
sink, but at worst that sinking will only be a few meters. If you live
very close to the beach and close to sea level then there is a
possibility that your house will be flooded, but no big piece of the
island is going to break off and disappear. It is possible that there
will be submarine landslides along the steepest slopes offshore, but
each of those is likely only be small in extent.
You should understand how to protect yourself in an earthquake. You
should also work out evacuation routes to get away from the ocean to
higher ground--if there is a big earthquake along your coast then the
risk of a tsunami (tidal waves) is high and you'll probably only have
ten minutes to evacuate. Figure out either the fastest way to get above
an elevation of 20 metres (the top of a tall sand dune is fine) or to
get a kilometer or so inland from the shoreline. Make sure everyone in
your household knows the evacuation route. If the ground shakes so
severely that you have trouble standing, as soon as the creaking and
rattling dies down you should immediately make for safer ground--that
is something that everyone must understand. A large number of people
who die in tsunamis do so because they go back to check on Granny or to
get the kids out--you should be able to trust Granny and the kids to
look after themselves. If you do have a tsunami, remember that the
shoreline is likely to be hazardous for several hours.
I hope this message is not too alarming. At least I can reassure you
that Vancouver Island is not going to break apart in the next big
earthquake. Rest assured too that a lot of people are trying to figure
out just how Cascadia earthquakes happen so that they can advise you of
the risks and maybe even give you a warning when one is about to
occur.